What's a GPT?

by Rod Smith, rodsmith@rodsbooks.com

Last Web page update: 4/18/2022, referencing GPT fdisk version 1.0.9

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Note: This page is part of the documentation for my GPT fdisk program.

In 2022, the vast majority of new x86-64 (aka AMD64 or X64) computers in stores come with GPT disks. This was not always the case, though, and it's not true of all disks even today. If you've been using computers since before about 2011, or if you've ever used a removable disk, you may be familiar with older disk partitioning schemes — or perhaps you're not familiar with them, and don't understand why older computers or removable disks are partitioned in a different way from internal modern disks. This page should help elucidate these differences, and why the shift to GPT in the arly 2010s was so important.

MBR, Its Annoyances, and Its Limits

Since the earliest hard disks for x86 computers, these disks have been divided into one or more partitions using a partitioning scheme that has, through the ages, gone by several names, such as MS-DOS disklabels, BIOS partitions, and Master Boot Record (MBR) partitions. This partitioning system squeezed both a boot loader and partition table into the first sector of the disk. It was simple, but by whatever name, the MBR partitioning scheme has several characteristics that have been limitations or annoyances:

There are techniques you can use to extend the life of MBR; however, these methods are stop-gaps at best. Sooner or later, you'll find MBR to be inadequate as you move to larger and larger disks.

GPT to the Rescue

MBR's clear successor is GPT. This new partitioning scheme fixes many of MBR's problems:

Unfortunately, GPT is not without its problems. These mainly relate to compatibility, such as:

To protect GPT disks against errant older disk tools, GPT keeps an MBR partition table on the first sector of the disk. This MBR contains a single disk-spanning partition of type 0xEE, which makes older tools think the disk is in use by an unknown OS. Some tools take advantage of this feature to create a hybrid MBR configuration, in which some partitions are accessible via both GPT and MBR definitions. Although this is non-standard, awkward, delicate, and downright dangerous, it can help make the transition from MBR to GPT easier by providing a workaround for OSes that don't understand GPT.

In addition to the protective MBR, GPT features two main types of data structure, each of which is stored on the disk twice:

Because of the hard 2 TiB limit of MBR partitions, chances are you'll be forced to switch to GPT for at least some disks in the not-too-distant future, assuming you haven't yet done so. MBR is likely to remain useful on smaller devices, such as USB flash drives, for years to come. In 2022, many older x86 and x86-64 PCs still use MBR-partitioned disks, although as noted earlier, UEFI-based PCs are now the standard for new computers. If an OS boots in EFI mode, chances are its disks use GPT.

Go on to "Working Around MBR's Limitations"

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copyright © 2009–2022 by Roderick W. Smith

If you have problems with or comments about this web page, please e-mail me at rodsmith@rodsbooks.com. Thanks.

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